Tanzanian Exclusive Economic Zone (EEZ)

1: Yellowfin Tuna (Highly Migratory)

Yellowfin Tuna (Highly Migratory)

Yellowfin Tuna (Highly Migratory)

2: Indian Ocean Bigeye Tuna (Deep-Diving, Overfished)

Fishery: - Caught by longlining (deep hooks target night feeders) - High value for sashimi market (Japan, SE Asia) - Tanzania vessels and foreign fleets participate - Increasing fishing pressure in recent decade

Status: - OVERFISHED according to latest IOTC assessments - Stock experiencing recruitment overfishing - Fishing mortality exceeds target reference points - International concern about sustainability

Management Issues: - Growth to high value before reaching maturity - Young fish size matches market preference - Catches may contain immature individuals - Difficult to monitor deep-diving behavior - Data-limited assessments due to depth complexity

Case Study 3: Indian Ocean Swordfish (Straddling)

Species: Swordfish (Xiphias gladius)

Swordfish Illustration

Distribution Pattern: - Throughout Indian Ocean - Some stocks straddle between: - Tanzanian EEZ - Mozambican EEZ - High seas regions

Fishery: - Caught by long-line vessels (surface and deep long-lines) - Mixed catch with other tunas - Important for artisanal fisheries in coastal areas

Management Issues: - Some populations overfished historically - Requires coordination between Tanzania and Mozambique - IOTC sets recommendations but enforcement challenging - Difficult to distinguish stocks at sea

Case Study 4: East African Kingfish and Snappers (Transboundary)

Species: Kingfish (emperors, jobfish) and red snappers

Distribution: - Demersal species (bottom-dwelling) - Found along continental shelf - Occur in both Tanzanian and Kenyan EEZs - Limited high seas component

Characteristics: - Transboundary stock (occurs in 2+ EEZs) - NOT highly migratory - More localized movement - Important for both commercial and artisanal fleets

Management: - Less formal international coordination - Tanzania and Kenya set own regulations - Bilateral discussions through regional bodies - Lake Victoria has similar transboundary stocks (Nile Perch)

Case Study 5: Small Pelagics (Sardines and Anchovies) - Straddling

Species: - Sardines (Sardinella species) - Anchovies (Engraulis species)

Distribution: - Occur in coastal zones - Some stocks straddle Tanzanian-Mozambican border - Influenced by Indian Ocean upwelling systems - Seasonal migrations with monsoon currents

Fishery: - Primarily artisanal and semi-industrial - Important for food security - Canned fish export product - Growing demand from aquaculture feed

Management Challenge: - Straddling populations with limited coordination - Tanzania and Mozambique have weak management data sharing - Environmental variability affects distribution - Recruitment highly variable (climate-dependent)

Tanzania’s Management Framework

National Level: - Ministry of Livestock and Fisheries sets regulations - Licensing system for foreign vessels - Catch monitoring and reporting requirements - Port state measures to prevent IUU fishing

Regional Level: - IOTC member - Complies with tuna management - SWIOFC (Southwest Indian Ocean Fisheries Commission) - Nairobi Convention for shared environmental concerns

International Level: - Bilateral agreements with neighbors (Kenya, Mozambique) - Participation in IOTC decisions - Port state inspection of foreign vessels - Market-based measures (catch documentation)

Key Challenges for Tanzania

1. IUU Fishing: - Foreign vessels fishing without permits - Catch not reported to IOTC - Undermines quota system - Reduces resource access for legitimate fishers

2. Limited Monitoring, Control, and Surveillance (MCS): - Few patrol vessels available - Large EEZ difficult to monitor - Weak port state infrastructure - Limited satellite monitoring systems

3. Data Limitations: - Incomplete catch reporting from artisanal sector - Species identification challenges at sea - Limited biological sampling programs - Weak data sharing with neighbors

4. Stock Mixing: - Cannot distinguish stocks at sea - Straddling populations require coordination - Enforcement of bilateral limits difficult

Regional Cooperation Initiatives

IOTC Resolutions affecting Tanzania: - Yellowfin tuna catch limits (annual review) - Bigeye tuna closure during spawning season - Vessel monitoring systems (VMS) requirements - Port state inspection protocols

SWIOFC Initiatives: - Shared research on migrant stocks - Harmonized data collection protocols - Capacity building for stock assessment - Training in compliance monitoring

Bilateral with Mozambique: - Joint assessment of shared demersal stocks - Coordinated licensing in border areas - Shared research surveys - Joint enforcement patrols (periodic)

Summary: Tanzania Context

Highly Migratory Stocks in Tanzanian Waters: - Yellowfin tuna, bigeye tuna, swordfish - Managed through IOTC - Require international compliance - Subject to scientific stock assessments

Straddling Stocks: - Small pelagics, some demersal species - Require bilateral coordination - Limited formal management - Data sharing challenges

Transboundary Stocks: - Kingfish, snappers in shared EEZs - Bilateral management with Kenya - Weaker international framework - Growing importance for food security

Management Lesson: Fish migration patterns require management cooperation that extends beyond national borders. Tanzania’s challenge is balancing: - National fishing industry needs - International obligation compliance - Regional cooperation requirements - Limited resources for monitoring and enforcement

Priority Fisheries in Tanzania

Tanzania’s fisheries are strategically important for food security, employment, and economic development. Priority ranking considers: - Economic value (export revenue) - Food security contribution (protein supply) - Employment (fisher livelihoods) - Management feasibility (data availability)

Priority 1: Indian Ocean Tuna (Yellowfin, Bigeye, Skipjack)

Economic Importance: - Highest export value (~$40-60 million annually) - Large foreign fleet participation (revenue from licenses) - International market: Japan, SE Asia, EU - Premium sashimi prices drive fishing pressure

Current Status: - Yellowfin: Fully exploited (IOTC assessments) - Bigeye: Overfished (recruitment concerns) - Skipjack: Lightly exploited (increasing catches)

Management Structure: - IOTC sets scientific advice - Tanzania implements catch limits through: - Vessel licensing system - Landing requirements - Port state measures

Challenges: - IUU fishing reduces legitimate catch opportunities - Data reporting gaps (underreporting common) - Limited monitoring capacity for high seas - Seasonal catchability variations

Priority Actions: 1. Strengthen vessel monitoring systems (VMS) 2. Improve port state inspection protocols 3. Enhance catch documentation compliance 4. Support scientific observer programs on vessels 5. Develop capacity for onboard monitoring

Priority 2: Small Pelagics (Sardines, Anchovies)

Food Security Importance: - Essential protein source for coastal communities - ~15-20% of national fish production - Affordable for low-income households - Direct human consumption (not reduction fishery)

Current Status: - Stocks: Moderately exploited to fully exploited - Fluctuating catches (environmental variability high) - Limited biological data - Data-limited stock assessment conditions

Fishery Characteristics: - Artisanal and semi-industrial vessels - Gill nets, beach seines common gear - Peak season: June-September (southwest monsoon) - Limited export (mostly domestic consumption)

Management Issues: - Weak regulatory enforcement - Poor catch statistics from artisanal sector - Straddling stocks (shared with Mozambique) - Climate vulnerability (upwelling dependent)

Priority Actions: 1. Implement length-based assessment methods (LBSPR) 2. Improve artisanal catch monitoring (fisher interviews) 3. Establish landing sites with standardized sampling 4. Bilateral coordination with Mozambique 5. Climate-informed management strategies

Priority 3: Demersal Fish (Kingfish, Snappers, Emperors)

Fishery Importance: - Economic value: Medium ($20-30 million) - Food security: Important for protein supply - Employment: Primarily artisanal fisher groups - Cultural significance in coastal communities

Current Status: - Many stocks: Overexploited (catch declining despite effort) - Size structure: Skewed toward younger/smaller fish - Growth overfishing evident (young fish caught) - Reproductive capacity reduced

Fishing Methods: - Long-lines (commercial vessels) - Hand-lines (artisanal fishers) - Small seines (coastal communities) - Traps (specialized gear)

Management Structure: - National Ministry of Livestock and Fisheries sets regulations - Bilateral with Kenya (transboundary stocks) - Limited IOTC involvement (not highly migratory) - Data-limited assessments typical

Priority Actions: 1. Implement minimum size limits (protect juveniles) 2. Develop sustainable fishing practices training 3. Establish marine protected areas (MPAs) for spawning 4. Improve landing site monitoring 5. Gear selectivity improvements (escape gaps)

Priority 4: Lake Victoria Fish Stocks (Nile Perch, Tilapia)

Fishery Importance: - Food security: ~10% of national fish production - Employment: Important for inland fishing communities - Regional trade: East Africa (Kenya, Uganda) - Export potential: Limited (local species)

Current Status: - Nile Perch: Overfished in some areas, recovering in others - Tilapia: Depleted stocks in many zones - Cichlid species: Declining biodiversity - Ecological restructuring from invasive species

Management Context: - Shared: Tanzania, Kenya, Uganda - Lake Victoria Fisheries Organization (LVFO) - Complex governance (multiple countries, agencies) - Transboundary challenges significant

Priority Actions: 1. Strengthen LVFO coordination mechanisms 2. Implement area-based management plans 3. Improve data collection infrastructure 4. Support artisanal fisher livelihoods (alternative incomes) 5. Environmental monitoring (pollution, climate impacts)

Priority 4B: Lake Tanganyika Fish Stocks

Lake Context: - World’s second-deepest freshwater lake (1,470 m) - Shared by 4 countries: Tanzania, DRC, Burundi, Zambia - 68,800 km² surface area - Unique cichlid-dominated ecosystem (>1,000 endemic species)

Fishery Importance: - Food security: ~5-10% of national fish production - Employment: Important for inland fishing communities (especially Kigoma region) - Regional trade: Dried fish to DRC, Kenya - Economic value: ~$10-15 million annually - Protein source for ~1 million people around lake

Key Fish Stocks: - Cichlids (Lates niloticus - Nile perch, Clarias species) - Nile perch: Introduced 1950s, now dominant predator - Clarias catfish: Traditional species, declining - Native cichlids: Drastically reduced by Nile perch predation

• Sardine-like fish (Limnothrissa, Stolothrissa)
  • Small pelagics forming basis of food web
  • Traditional food fish (dagaa equivalent)
  • Seasonal variations (water mixing)

Current Status: - Nile perch: Overexploited in northern basin - Size structure declining (fewer large individuals) - Recruitment variable (climate dependent) - Recovery potential unclear

• Cichlid diversity: Severely depleted
  • Biodiversity loss continues
  • Ecosystem function altered
  • Native species near extinction risk
• Pelagic fish: Variable productivity
  • Oxygen minimum zone expanding (climate change)
  • Recruitment highly variable
  • Catch fluctuations 50-100% year-to-year

Fishery Characteristics: - Primarily artisanal vessels (small boats, dugout canoes) - Gill nets, hand-lines, traps common gear - Seasonal fisheries (migration, water temperature patterns) - Multi-species, multi-gear fishery (very mixed) - Limited cold chain (dried/salted products)

Management Structure: - Shared by Lake Tanganyika Catchment Management Project - Limited formal international body (unlike Lake Victoria with LVFO) - National regulations by each country (inconsistent) - Data exchange minimal between countries - Transboundary coordination weak

Challenges: - Overfishing: Catch-per-unit-effort declining - Data deficiency: Poor catch statistics (artisanal sector) - Limited biological knowledge: Few stock assessments - Pollution: Mining, agriculture runoff affecting water quality - Climate impacts: Oxygen depletion, temperature shifts - Invasive species: Nile perch interactions with natives - Governance gaps: No Lake Tanganyika Organization equivalent - Regional conflicts: DRC illegal fishing in Tanzanian waters

Priority Actions: 1. Establish Lake Tanganyika Fisheries Commission (formal structure) 2. Conduct baseline biodiversity and stock surveys 3. Implement artisanal catch monitoring (beach landing sites) 4. Develop minimum size regulations with region-wide coordination 5. Create marine protected areas (MPAs) in deep basins for spawning

Unique Considerations: - Deep-water ecosystem: Complexity of thermal stratification - Anoxic zone: Limited suitable habitat at depth - Productivity: Lower than Lake Victoria (oligotrophic) - Endemic species conservation: Global biodiversity significance - Climate vulnerability: Warming rates higher in tropics - Livelihood diversity: Need alternative income sources

Priority 5: Emerging Fisheries (Cephalopods, Deep-Sea Fish)

Development Status: - Octopus fishery: Growing (traditional + commercial) - Deep-sea shrimps: Limited exploitation - Unexploited species: High seas potential

Economic Potential: - Octopus: High export value (~$5-10 million) - Specialty markets: Europe, Japan - Employment: Coastal communities (seasonal)

Current Status: - Limited biological knowledge - Data-deficient stocks (IUCN Red List classifications) - Catch statistics incomplete - No formal stock assessment

Management Needs: 1. Develop baseline biological surveys 2. Establish catch monitoring systems 3. Set precautionary catch limits 4. Environmental impact assessments 5. Community-based management trials

Management Priorities Matrix

Integrated Management Strategy

Short-term (1-2 years): - Improve catch monitoring across all sectors - Enhance data collection infrastructure - Build capacity in stock assessment methods - Strengthen port state controls

Medium-term (3-5 years): - Implement harvest control rules for priority stocks - Develop marine spatial planning - Establish management strategy evaluations (MSE) - Regional cooperation protocols

Long-term (5+ years): - Ecosystem-based fisheries management - Climate adaptation strategies - Sustainable livelihood diversification - Capacity for advanced stock assessments

Mixed stock

• A mixed stock fishery is a fishery stock of variety ages, sizes, species, geographic or genetic origins or any combination of these variables.

• It has some challenges in management due to difficulties in targeting a specific fish type using a certain fishing methods.

• Mixed stock of a species has significant differences in morphological and biological characteristics.

• The mixed stock population may unite into a unit stock by gene flow.

Mixed stock…

• The morphometric characters and meristic characters are to be taken for different stocks/populations of a species.

• The population are to be segregated using PCA statistical test

• Mixed stock of a species is common in tropical waters.

• Assessment of such tropical stocks becomes difficult by using conventional models.

Mixed stock…

• Refinements are needed for proper assessment.

• The aging becomes difficult in a mixed stock of tropical species.

• Usually many cohorts are released from a tropical stock; hence the assessment becomes a problem on tropical stocks of a mixed species.

• Mixed stock of a species has varied spawning seasons and the calculation of length at first maturity and growth gets varied

Limitations in the assessment of tropical fish stocks

• For calculation of stock assessment of mixed stock of a species, the conventional models framed for temperate species should be used in caution.

• The mortality parameters such as F and M, fishing effort and growth parameters are to be calculated separately

• Because these parameters form input data for estimation of stock in conventional models.

Part 2: Methods of Estimating Fish Stock Abundance

2.1 Direct Methods (Surveys and Sampling)

Trawl Surveys: - Bottom or mid-water trawling in systematic grid pattern - Standardized vessels, gear, and protocols ensure comparability - Provides age/size composition data - Index of relative abundance (CPUE) - Advantages: Direct observation, compositional data - Disadvantages: Gear selectivity, avoidance behavior, high cost

Dredges and Dip Nets: - For benthic and demersal species - Smaller spatial coverage than trawls - High observation effort per area

Seining and Gillnetting: - Specialized methods for coastal and nearshore areas - Often used in smaller-scale fisheries

2.2 Catch Per Unit Effort (CPUE) Indices

Definition: CPUE is the catch obtained per standardized unit of fishing effort.

Formula: \[\text{CPUE} = \frac{\text{Total Catch}}{\text{Total Effort}}\]

Where effort could be: - Fishing hours - Days at sea - Number of hooks (for longline) - Distance trawled - Number of traps

Standardization: - Raw CPUE must be standardized for differences in: - Vessel characteristics (size, power, technology) - Seasonal effects - Geographic area - Environmental variables - Generalized Linear Models (GLM) or other statistical methods used

Advantages: - Uses existing commercial fishing data - Cost-effective - Long time series available - Real-time updates possible

Disadvantages: - Affected by fisher behavior (targeting, avoidance) - Changes in technology affect catchability - Assumes effort is independent

2.3 Mark-Recapture and Tagging Studies

Basic Principle: 1. Capture, mark, and release a known number of fish (N₁) 2. Recapture a sample (n₂) 3. Count marked individuals in recapture (m) 4. Estimate population size using Lincoln-Petersen formula:

\[\hat{N} = \frac{N_1 \times n_2}{m}\]

Modified Approaches: - Multiple recapture events (Schnabel method) - Robust design (closed sessions within open population) - Bayesian approaches for uncertainty quantification

Advantages: - Direct abundance estimate for specific area - Provides movement and survival information - Useful for smaller populations

Disadvantages: - Tagging causes stress and mortality - Tag loss reduces accuracy - Expensive and labor-intensive - Limited to feasible population sizes

2.4 Acoustic Surveys

Principle: - Sound waves detect fish schools through differences in density - Returns are converted to fish density estimates - High spatial resolution (meters)

Technology: - Single-beam and split-beam echosounders - Multibeam systems for 3D mapping - Autonomous underwater vehicles for remote surveys

Target Strength: - Measured in dB (decibels) - Relates to fish size and species - Must be calibrated for reliable estimates

Advantages: - Rapid spatial coverage - Non-lethal - Good for pelagic fish schools - Can estimate fish size distribution

Disadvantages: - Species identification challenging - Shallow water complications (near-field effects) - Environmental noise affects detection - Expensive equipment

2.5 Visual Surveys and Underwater Observation

Snorkeling and Diving: - Direct observation of fish density - Limited to clear water and shallow depths - Good for reef or estuarine species - High survey cost per area

Underwater Video: - Remote operated vehicles (ROVs) for deeper water - Fixed cameras at monitoring stations - Reduces observer effects - Good for benthic species

Aerial Surveys: - Visual observation from aircraft - Limited to shallow, clear water - Fast coverage of large areas - Good for surface schooling species

Advantages: - Direct observation (reduced assumptions) - Species identification possible - Behavioral information

Disadvantages: - High labor cost - Limited to specific conditions - Depth and visibility constraints

2.6 Population Estimation Models

Statistical Catch-at-Age Models: - Fit population models to catch and survey data - Estimate population numbers by age and time - Account for fishing mortality and natural mortality - Provide uncertainty estimates

Integrated Assessment Models: - Combine multiple data sources (catch, CPUE, surveys, biology) - Increase precision of estimates through data integration - Common models: CASAL, Stock Synthesis, ADMB

Bayesian Approaches: - Incorporate prior knowledge about population parameters - Provide posterior distributions of population estimates - Better uncertainty quantification

Virtual Population Analysis (VPA): - Back-calculate cohort abundance from catch and natural mortality - Limited to data with strong age structure

2.7 Biomass Indices and Standardized Indices

Simple Biomass Indices: - Aggregate CPUE across fleets or time periods - Index of relative abundance - Assumes catchability constant

Standardized Indices: - Remove effects of: - Seasonal variation - Vessel and gear differences - Environmental variables - More reliable trend indicators

Abundance Indices vs. Biomass Indices: - Abundance index: Number of fish per unit effort - Biomass index: Weight per unit effort - More relevant for fisheries management - Accounts for size differences

Index Combination: - Multiple indices increased reliability - Weighted by inverse variance - Accounts for different spatial and temporal coverage

Conclusion

Estimating fish stock abundance requires understanding both the biological characteristics of the stock and the strengths/limitations of available methods. No single method is perfect; successful stock assessment typically combines multiple approaches to maximize accuracy and precision of estimates.

References and Further Reading

• Quinn, T. J., & Deriso, R. B. (1999). Quantitative Fish Dynamics. Oxford University Press.
• Hilborn, R., & Walters, C. J. (1992). Quantitative Fisheries Stock Assessment. Chapman & Hall.
• Petitgas, P. (1998). Geostatistics in fisheries survey design and stock assessment. FAO Fisheries Technical Paper 368.

Last updated: December 2025